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Full text of "Recent developments in physics"

RECENT DEVELOPMENTS IN PHYSICS. 



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RECENT DEVELOPMENTS IN PHYHICS. 



THE NATURE OF LIGHT EMISSION. 

IT would be difficult to find a man on the street who has not 
heard of wireless telegraphy and electric waves. Few men, 
however, realize that their own eyes are constantly receiving 
wireless messages, that light waves are electrical in nature. 
When electric waves are sufficiently small the eye becomes 
their detector, and we have the sensation of light. The long 
waves sent out by the wireless operator on board ship are but 
the big brothers of the light waves emitted by a luminous 
body. It is only within the last fifty years, however, that 
this fact has been brought to light. In 1865, J. Clerk Maxwell 
as a result of brilliant mathematical analysis was led to enun- 
ciate his "Electromagnetic Theory of Light." Thirteen years 
later the theory received striking experimental confirmation. 
Hertz, experimenting on electric waves in his laboratory, 
showed that these waves could be reflected, refracted, polar- 
ized and in other respects conducted themselves exactly like 
light waves. The brilliant work of Hertz left little doubt of 
the truth of Maxwell's theory, and since his time investigation 
has but confirmed it. At the present time the theory is 
universally accepted. 

The physicist of the twentieth century is directing his at- 
tention to what is going on at the source of light. Just what 
is the nature of light emission? Of what nature is the vi- 
brating body sending out the minute electric waves ? Are its 
vibrations subject to mathematical analysis? These and 
other questions he is seeking to answer. In this note a short 
outline of the present position is given. 

It will be noted first of all that if we accept the theory 
that light waves are electrical in nature, we must have an 
electrical source. This has been supplied for us by work in 
other fields of Physics. In another article in this number 
of the Quarterly, it is pointed out that atoms of all substances 
contain definite numbers of electrons, negatively charged par- 
ticles of very small mass. It is in the vibrations of these 
electrons that we find our source of light waves. Sometimes 



498 QUEEN'S QUARTERLY. 

the atoms of substances are not allowed to pursue the even 
tenour of their way, — a little common salt is put in a gas 
flame, or an electric discharge is sent through a tube contain- 
ing a rarefied gas — a commotion takes place among the elec- 
trons, they are set vibrating and short electric waves are sent 
out. These, falling on our eyes, give us the sensation of 
light. 

We must refer next to the question of the analysis of the 
light emitted by any luminous body. If we pass ordinary 
sunlight through a glass prism (a spectroscope) we find an 
emergent "rain-bow" of light with colors running from red to 
violet. If, however, we examine the light emitted when an 
electric spark passes between, say, two pieces of iron, we now 
find a spectrum consisting of a great many coloured lines, each 
distinct from the other. The same result is obtained when 
we analyze with our prism the light emitted by a gas made 
luminous by an electric discharge. It has been shown, fur- 
ther, that these spectral lines occur in series, that a definite 
mathematical relation obtains with reference to the positions 
they occupy in the spectrum. In other words, if we know 
the position of one line, from a general mathematical expres- 
sion we can calculate the position of the, others. The explan- 
ation of this realm of law concerning spectral lines is evident 
if we remember that the prism simply analyzes into simple 
components the very complex vibrations within the atom. 
Each spectral line corresponds to a simple vibration, a great 
number of which make up the actual resultant vibration at 
the source. 

We have referred to work on spectral series because it is 
closely related to one of the outstanding problems concerning 
the minds of physicists at the present day. This problem is 
just the converse to the above. Is it possible to take the atom 
with its electrons and mathematically predict what spectral 
lines should be emitted ? Can we analyze mathematically the 
vibrations sent out by a disturbed electron and show that the 
components bear the same ratio to each other as that obtain- 
ing among spectral lines? This is the problem now demand- 
ing solution at the hands of mathematical phisicists.. Already 
considerable advance has been made. Within the last year 
papers have appeared in several of the leading physical jour- 
nals, in which it has been shown that for simple atoms, at any 



RECENT DEVELOPMENTS IN PHYHICS. 499 

rate, the problem is soluble. Undoubtedly the near future 
will show marked advances in this field of enquiry. 

The problem of the nature of light emission is being at- 
tacked also by experimental physicists, the most prominent of 
whom is Prof. Wood of Johns Hopkins University. For some 
years he has been investigating the fluorescent properties of 
vapours. The general reader will recall that certain sub- 
stances emit light when stimulated by light from some exter- 
nal source. They are said to fluoresce, or, — if the light con- 
tinues to be emitted on removal of the outside source — to phos- 
phoresce. It is possible, therefore, to cause bodies to emit 
light under the stimulus of an external "exciting" source. The 
general reason for this is evident. The exciting light waves, 
being electrical in nature, set into vibration the electrons 
within the atoms of the substance on which they fall, thus 
causing a light emission from the latter. Now it will at 
once be clear that for fluorescence to take place, there must 
be some sort of "tuning" between the frequency of the vibra- 
tions falling on an atom and the natural frequency of the 
vibrations which itself would emit. In other words we cannot 
expect to obtain fluorescence for all vibrations which may fall 
on a body, and experiment shows that this is the case. As 
both the exciting light and the fluorescent light may be 
analyzed with a spectroscope, we have a means of examining 
any relation which may exist between them. We are thus 
supplied with another method of attacking the problem of the 
nature of light emission. 

In this field Prof. Wood has carried out already several 
brilliant researches. His work has been confined to a study 
of the fluorescent properties of vapours, substances which, on 
account of their comparative simplicity of structure, render 
the problem much less complex. He has found tliat even the 
slightest variation in the nature of the exciting vibrations, 
produces marked differences in the fluorescent light. By 
controlling the vibrations of the source within very narrow 
limits, he hopes to throw considerable light on our knowledge 
of the internal vibrations of the atom. The brilliant work 
he has accomplished already is a voucher that much may be 
expected from him in the future. 

J. K. R. 



500 QUEEN'S QUARTERLY. 

THE QUANTUM THEORY. 

One of the greatest triumphs of modern Physics is the 
firm establishment of the atomicity of matter ; that matter is 
built up of definite sized units. Ostwald, the great German 
chemist, and his band of followers were for many years the 
chief opponents of this theory, but in face of the accumulated 
evidence collected by the physical chemists, Ostwald was led to 
accept the atomic theory. Now, we are able to count the 
molecular units in a given mass of matter, measure their size 
and follow their movements. We have learned also, that 
instead of being a simple thing, as originally supposed, the 
atom has electrical constituents. Then again, it has been 
discovered that the electrical part of the atom of matter has 
itself an atomic structure. The value of the charge of an 
elementary electrical unit has been definitely measured in 
many ways, and has been found invariable. 

The latest atomic theory deals with energy and is called 
the quantum theory. Of course, we all recognize electricity 
as a manifestation of energy and now scientists place matter 
in the same category ; so that the quantum theory is really the 
old atomic theory generalized to apply to all kinds of energy. 
It was first proposed by Planck about ten years ago in order 
to account for the way in which heat energy leaves a heated 
body. Many attempts had previously been made to develop 
an equation expressing the relation between the temperature 
of a body, and the rate at which heat left it, or in other words 
its rate of cooling. Newton, Du Long and Petit, and various 
others had worked out empirical f ormulae,which. however, held 
only for small ranges of temperature. Wien and Rayleigh, 
mathematical physicists, had each, on theoretical grounds, de- 
veloped a formula, which was found incorrect when tested by 
experiment. The only assumption they had made was that 
heat energy is continuously emitted by a hot body. Planck 
attacked the same problem and assumed from the beginning, 
that the emission of energy from the heated body is not con- 
tinuous, as they supposed, but is sent off at intervals in small 
units or so called "quanta." From this he deduced a relation 
which, contrary to all the other ones, agreed well with the 
experimental facts. Since then Planck's idea has been applied 
to all kinds of interchanges of energy. Nerst, a chemist, has 



RECENT DEVELOPMENTS IN PHYH1CS. 501 

shown that the quantum theory will account for the way in 
which the specific heats of bodies change as they are cooled 
to very low or heated to very high temperatures, and Einstein 
and J. J. Thomson have formulated a quantum theory of light 
which is radically different from the older wave theory. The 
Einstein-Thomson theory is that light, instead of being given 
out continuously by an illuminated body and propogated in the 
form of waves through the ether, is sent off from the body by 
means of energy spots, each spot containing an integral num- 
ber of energy units or quanta. Accordingly, a beam of light 
is not a continuous wave structure but consists of energy spots 
distributed at random through it, all of which are moving with 
the same velocity. It is rather remarkable that this theory 
resembles Newton's corpuscular theory which was abandoned 
in favour of the wave theory. 

In view of recent work, it is necessary to adopt an atomic 
or quantum theory with regard to X-rays as opposed to a wave 
theory. An X-ray pulse produced by the stopping of a 
cathode particle hurls out an electron from a molecule. It is 
found that the amount of energy required to throw an electron 
from a molecule is the same as that required to stop the 
electron which produced the X-ray. The X-ray, then, must 
simply transfer energy from one electron to another. To do 
this, it is apparent that the X-ray cannot be a spherical wave 
pulse which spreads from the point where it is made. Rather, 
it must be a form of localised energy which travels directly 
to the place where it ejects an electron. Only in this way 
could the same amount of energy be transferred from the place 
where the X-ray is made to the place where it is used up in 
driving an electron from a molecule of matter. The X-rays 
then must have an atomic structure. 

When we come to consider the late experiments of Laue, 
Friedrich, and Knipping, and those of Bragg and others, we 
find that X-rays are very similar to ordinary light rays in their 
properties. It is most probable then, that ordinary light is 
built up in the same way as X-rays, and that Einstein and 
Thomson's quantum theory of light is correct,, 

Whatever the ultimate fate of the 'quantum theory, it 
seems probable that it will do its part in pointing the way to 



5 o2 QUEEN'S QUARTERLY. 

new investigations. We have tried to show how it has co- 
related experimental data, which no other theory could do. 
Any theory is strong in proportion to its power to do these 
things and the quantum theory seems to have won its place 
among the other theories of natural phenomena, when judged 
by such a standard. 

V. E. P.